Machine learning based query expansion methods tackle CQU of Equation REF  by defining a binary term classification problem: for each term appearing in dialog history \(q \in \mathcal {H}_i\) , decide whether to add it to the current query \(Q_i\) . That is, \(\mathcal {S}^*\)  consists of all the terms selected by a binary term classifier learned from training data  [1]}, [2]}, [3]}.
